Magnetic recording and reproducing circuits



Feb. 5, 1957 L. J. BOBB MAGNETIC FECORDING AND REPRODUCING CIRCUITS Original Filed Nov 24, 1947 INIENTO m ATTORNEYS d 2 ,7 80,67 8 [C6 Patented: Feb-.-5., 1957 MAGNETIC RECORDING AND REPRODUGING CIRCUETS Lloyd J. Babb, Glenside, Pa., assignor to The International Electronics Company, Philadelphia, Pa, a corporation of Pennsylvania Original application November 24, 1947, Serial No. 787,645,. now Patent No. 2,600,046, dated Junev 10, 1952,. Divided and this application May 8, 1952, Serial No. 291,998

1 Claim. (Cl. 1791il0.1)

No 2,600,046, and relates to the circuits of ofiice dictation equipment of the type disclosed in copending application Serial No. 717,754, filed December 21, 1 946, and assigned to the assignee of the present invention; and copending application Serial No. 11,993, filed February 28, 19.48, assigned to theassignee of the present invention and entitled. Magnetic Recording Machine, although it is to be. understood that the invention here disclosed is not limitedto use with the apparatus of the said copendi'ng applications.

A general object of the present invention is to provide a simple and efficient arrangementol circuit elements for use. in magnetic recording and reproducing: apparatus.

A morepanticular object of the invention isto simplify 'theoperation ofmagnetic recording and reproducing apparatus.

A stillturthe-r objectis toeliminatethe necessity for readjusting theuamplification of the apparatus each time a; change is. made from recording to reproducing conditions;

A. furthenohject is. toprovide. for separate control of,

the amplification of the system during recording and reproducing.

Yeta-nothenobject. of theinuention isto. provide for flexible; interchange: ot-auxiliary. equipment. usedv with the circuit, but hare included certainelements ofthe mechanical assembly of a magnetic recording and reproducing system of, the-type, disclosed in the copending application last abovomentioned- In the drawing-,1 have, indicated by the dotted line A the division between the mechanical assembly and1theelect-ronic chassisof a magnetic, recording; system, the'mechanical assembly being representedby that: portion of. the drawing above the line. A. I have also includedin;the-electronicchassis.a terminalblock towhich various;ex-ternal.= connections may bemade. This block. is

bounded: in the. drawings by the dotted lines. B and C;

Considering first the elements of the mechanical-assembly, attention is directed to a magnetic; recording and reproducing head 10, an-era-singhead11, a solenoid 12 tor. actuating backspacingmechanism, a second solenoid 13 f or controlling the. initiationand termination of. scanning ofja magnetic record, ,adirection-sensitive switch.;l4, and a magnetic record translating motor .15. Thcelements 1=0 through'lS' are adapted to be electrically interbe'made to one side of the power line.

connected. with the electronic. chassisby means: at separable connectors which may takeany convenient form, but which I have illustrated asseparate jacks, one for each conductor.

Turning now tothe amplifier and its associated circuits,

. it will be seenthat theiinput circuit includes a transiiormer '16, Whose secondary is connected to the grid of a vacuum tube V4. .By means. of .a switch. S.1, theoutput of V1 is adapted tobe applied to. the grid of V2-either directly, when the. switch is in the left-hand or listen position, or through a fixed resistor 17 and a volume. control potentiometer 18 when the switchis in the right-hand or dictate position. Similarly, the. output of V2 is adapted to be applied to. the grid of vacuum. tube V-3 through a switch .S -2 either directly, whenthe switch is. in the righthand position, or indirectly through a volume control potentiometer. 19 whenythe switch. is in the left-hand position. The output of V-3 is applied to the grid of 1-4 through, a fixed. resistor 291, whose purpose will be described hereinbelow. The. primary of an output transformer 21 isconnected in the, plate circuit. of V-4.v

The drawing also includes a fifth discharge. tube V5, arranged. in an oscillator circuit whose. outputis adapted to be fed to the signal grid of V-4- through a. variable coupling capacitor 22 simultaneously with the audio frequency signallfrom V-3. 7

As is more fully disclosed in my copending application Serial No. 787,644, filed November 24, 1947, and entitled Magnetic. Recording and Reproducing Circuits, now Patent No. 2,641,655, thepurpose of resistor 20 is to block the polarizing voltage. generated by the oscillator from the circuits associated with V-3.

V5 receives direct. current plate potential through a switch S3 only when the apparatus is adjusted for recording, and, therefore, it will be understood that no voltages of the polarizing frequency are fed to V-4 when the apparatus is adjusted for reproducing.

The filaments of the electron discharge tubes are adapted to be connected in series across the power line asindicated at 23.. A resistor 24' is inserted. in this circuit to reduce, the current through the. filaments to the proper value. A pilot lamp 25 is also placed across the power line at this point, and serves to indicate the condition of the apparatusas determined by the power switch, 26.

Direct current potentials for the vacuum tubes are obtained from a rectifier-filter system comprising a series resistor 27-, a metallic half-wave rectifier 28, for example, of the selenium disc type, a choke 29, and filter condensers 30 and 31. Direct currents for operation of the solenoids is obtained from a second rectifier 32.

Alternating current for the motor 15 is adapted to be derivedfrom the alternating current power linethrough the jumpers indicated at 33 and 34 when. the apparatus is connected to an alternating current main. While the amplifier and, the rectifier filter systems are adapted for operation from alternating or direct current lines, the motor of the mechanical assembly is preferably an alternating current motor, and I have therefore provided for operation of the motor from direct current power means through the use of an inverter, indicated by a block 35, which isadapted to be connected into the circuitin place of jumpers 3'3 and 34. This inverter is adapted to convert the directcurrent appearing, at the. power input plug 36 to alternating current for the operation of motor 15, While permitting the amplifier to be energized directly from the directcurrent mains. Thus, the amplifier supply is free from inverter ripple, which would otherwise complicate the circuits due to the high amplification required inmagnetijc reproducing systcms,

Duegto the, factthatthe power supply employed is a transformerless system, the. various. return circuits, must To avoid personnel hazards, therefore, return circuits are made through a common conductor which is connected to the chassis through a capacitor 37. In addition, because of the relatively high amplification required during reproduction, certain of the return circuits are made to the same physical point 38 to avoid interstage coupling effects due to potential drops along the conductor.

Operating potentials for the various electron discharge tubes are derived from the rectifier-filter system in the conventional manner, with the exception of the arrangement of the circuits associated with V4. The cathode circuit of V4 includes a cathode by-pass capacitor 39 providing a path for alternating currents from the common conductor to the cathode of V4. The direct cathode current flows through a cathode dropping resistor 46 and thence through a conductor 41 to a switch S4, which, when the switch is in the left-hand or listen position, is connected to the common conductor and thus provides grid bias for V-4 in the usual manner by virtue of the potential drop across resistor 40. When switch S-4 is adjusted to the dictate or right-hand position, however, the cathode return circuit is through a conductor 42, erase head 11, and thence by a second conductor 43 to the common conductor at the point 44. Thus, when the apparatus is adjusted for recording, the cathode current of V4 flows through erase head 11 and develops therein a magnetic field for saturating the magnetic recording medium. The potential drop introduced by head 11 increases the grid bias of V4 during recording and thus reduces the amplification of the signal during this phase of operation. It will be seen, therefore, that this method of obtaining recording current also achieves a reduction in the overall amplification during recording, which is desirable in view of the relatively large output of the external transducer as compared with the output of the magnetic head during reproduction.

In addition to the elements of the circuit so far discussed, I have provided input-and output inverting switches 8-5 and 8-6. As will be seen, one side of the primary of transformer 16 and of the secondary of transformer 21 is connected to the common conductor at point 38. In addition, one conductor from each of jacks 45 and 46, and one side of magnetic recording-reproducing head is connected to this point. The high side of the input transformer primary is connected to the moving contact of switch S5, which is adapted, when the apparatus is adjusted for recording, to be in connection with contact 47 of jack 4-6.. When switch 8-5 is adjusted to the play back position, the high side of the input circuit is connected to the high side of magnetic head 10 through conductor 48.

Similarly, the moving contact of switch 8-6 is connected to the high side of the output transformer secondary and is adapted, when the apparatus is adjusted for recording, to connect the output transformer to head 10 through a filter comprising capacitor 49 and resistor 50.

This filter is of the type disclosed in my copending application, Serial No. 787,644, filed November 24, 1947, and entitled Magnetic Recording and Reproducing Circuits, now Patent No. 2,641,655. When switch 8-6 is adjusted to the play back position, the high side of the output transformer secondary is connected to the contact 51 of jack 45 and the switch contact 52 of jack 46.

It should perhaps be noted at this time that the output end of the filter comprising capacitor 49 and resistor 50 is connected with the common conductor through a bleeder resistor 53, of very high resistance, whose function is disclosed in my copending application last mentioned.

Certain other elements disclosed in the drawing should now be described, including a back space switch 54, which is adapted to be inserted in the circuit of solenoid 12 to provide for control of the back spacing function of the equipment. This switch may be either a hand switch or a foot switch, and, as indicated in the drawing, is

removably connected with the apparatus. A second switch 55, which may also be either hand or foot operated, or mounted on a microphone, is separably connected to the apparatus in a manner to provide control of solenoid 13 which, as was above stated, controls the scanning mechanism of the magnetic recording and reproducing apparatus. I have also illustrated in the drawing a mute switch 56, which is adapted to shunt the output transformer secondary during the manipulation of certain of the controls to silence the apparatus. In addition, a pair of ganged switches 57 and 58 and a solenoid 59, together with direction-sensitive switch 14, perform certain control interlock functions which are disclosed in copending application Serial No. 11,993, filed February 28, 1948, assigned to the assignee of the present invention and entitled Magnetic Recording Machine. Since they play no part in the present invention, these elements are not further described herein.

According to the invention, switches S-1, S-2, 5-3, 8-4, 8-5 and 8-6 are adapted to be operated simulta neously to adjust the apparatus between the two conditions of recording and reproduction. The jacks 45 and 46 are arranged to provide for the use alternatively either of a single external transducer, which is adapted to operate both as a microphone and as a loudspeaker, or for the use of two external transducers, one of which performs each of these functions.

Considering first the simplest mode of operation of the apparatus, assume that a small loudspeaker-microphone of the moving coil type is connected to the apparatus by means of a plug inserted in jack 45. No plug is in jack 46, and, accordingly, switch contact 52 is in engagement with jack contact 47, and, therefore, contact 51 of jack 45 will be connected to both switches S5 and 5-6. The loudspeaker will, therefore, be connected alternatively to the input and output terminals of the amplifier in accordance with the position of these switches. In addition, the magnetic head it) will be connected to the opposite terminal. Assuming that the ganged switches be in the recording position, the loudspeaker will be connected to the input of the amplifier and will function as a microphone, the head will be connected to the output of the amplifier through switch section 8-6, the cathode return circuit of V4 will be through erasing head 11, with the result that this head will be energized, and oscillator tube V5 will be energized through switch section S3. Switch section 8-1 will connect the grid of V2 to volume control potentiometer 13, and switch section 8-2 will connect the grid of V3 directly to the output of V2. Since fixed resistor 17 is in series with volume control potentiometer 18, the relatively large signal from the external transducer may be effectively reduced to a value which will not overload V-2 and which will spread the desirable volume range out over the physical range of adjustment of potentiometer 18 to give non-critical control of the amplification.

When the apparatus is adjusted for reproducing, switch sections S-3 and 5-4 will deenergize oscillator discharge tube V5 and erasing head 11 respectively. Switch sections S-5 and 5-6 will invert the previously existing connections to the input and output of the amplifier. Switch section 5-1 will connect the grid of VZ directly to the output of V4, eliminating both fixed resistor 17 and potentiometer 18, while the grid of V-3 will be connected to the output of VZ through volume control potentiometer 19. As a consequence, amplification during the reproduction phase of operation is controlled in the grid circuit of V3, at which point the amplitude of the signal from the magnetic recording head, after having passed through two stages of voltage amplification, is at a level appropriate to non-critical control.

All of the above discussion was predicated upon the connection to the apparatus of a single external transducer. If, now, in addition to the loudspeaker connected through jack 45, a microphone or other transducer be connected to the apparatus by means of a plug inserted in jack 46, contacts 47 and 52 will be separated, with the result that jack 45 will no longer be in connection with the input circuit of the amplifier under either condition of adjustment of switches S-5 and 8-6. On the other hand, jack 46 is in connection only with the input circuit. Accordingly, by simply inserting a microphone in the jack 46, the operator of the equipment may provide for the use of specialized transducers for the two functions.

It should also be pointed out that the use of the two volume controls above described not only provides for the non-critical control of amplification when the amplifier is energized by signals of widely different amplitude, but also provides for the separate and permanent adjustment of the apparatus for each of the. two conditions, so that the operator of the apparatus, after having once found the most desirable adjustment of the apparatus, is not required to readjust each time he changes from dicta tion to play back and the reverse.

It should be pointed out that the control of amplification by one potentiometer located at the position of potentiometer 18 during both reproduction and recording is disadvantageous, as the signal from the magnetic head in this case would be too small for smooth control, while if the single control were inserted at the position of potentiometer 19, the signal from the speaker-microphone I would overload V-2 under some conditions of operation.

In addition to the advantages arising out of each of the features of the invention which have been discussed above, it will be apparent that the employment of separate controls for recording and reproduction is particularly important when the apparatus is adapted to be used with a plurality of external transducers. Thus, when shifting from operation with a speaker-microphone to operation with an additional separate microphone, it is only necessary, with the apparatus of the invention, to readjust the recording volume control.

It should also be noted that energizing the erasing head during the recording operation by inserting it in the cathode circuit of one of the amplifier discharge tubes concurrently reduces the amplifier output by increasing for the motor regardless of the nature of the electrical the grid bias on that tube, and, thus, in addition to providing an economical source'of energizing current for the head, further aids in altering the overall amplifier gain when shifting from one condition of operation to the other.

Thus, according to the invention, I have provided extremely flexible means for connecting the apparatus to external transducers, have arranged the power supply circuits for the motor and amplifier in a manner to provide direct current for the amplifier and alternating current energy available, and, finally, have coordinated with the input and output switching arrangements an integrated system for modifying the amplifier characteristics according to the operating condition selected. As a consequence, a magnetic recording and reproducing system constructed in accordancewith the invention is adapted to respond to a wide variety of operating demands under widely varying operating conditions.

I claim:

Recording and reproducing apparatus comprising an amplifier having an input terminal and an output terminal, means for connecting the apparatus with a speaker microphone, first and second circuit means connected with said connecting means, a record-reproduce switch comprising first and second switch means for alternatively connecting said first circuit means with said input terminal and said second circuit means with said output terminal, and a jack for receiving the plug of a separate microphone, said jack including contact means for connecting the plug to said first circuit means and further including third switch means responsive to'insertion of the plug to break the connection between said first circuit means and said connecting means.

References Cited in the file of this patent UNITED STATES PATENTS 2,228,641 Roser Ian. 14, 1941 2,305,681; Dunning Dec. 22, 1942 2,359,489 Proctor et al Oct. 3, 1944 

